Method for producing silica sol

ABSTRACT

The present invention provides a means capable of suppressing the formation of fine particles in a method for producing a silica sol. The present invention relates to a method for producing a silica sol, including synthesizing a silica sol by, in a reaction liquid containing an alkoxysilane or a condensate thereof, water, and an alkali catalyst, allowing the alkoxysilane or condensate thereof to react with the water in the presence of the alkali catalyst, wherein the alkali catalyst is not additionally supplied after the start of the synthesis until the finish time of the synthesis, and during 90% or more of the time between when 5 minutes have elapsed from the time point when the electrical conductivity of the reaction liquid reaches a local maximum for the first time and the finish time of the synthesis, the proportion of the value of the electrical conductivity of the reaction liquid is more than 90% relative to the value of the electrical conductivity at the time when 5 minutes have elapsed from the time point when the local maximum is reached.

CROSS-REFERENCE TO RELATED APPLICATION

The present patent application is based on Japanese Patent ApplicationNo. 2019-064654, filed on Mar. 28, 2019, the entire disclosure of whichis incorporated herein by reference.

BACKGROUND 1. Technical Field

The present invention relates to a method for producing a silica sol.

2. Description of Related Arts

Conventionally, as a method for producing a silica sol, a productionmethod in which a sodium silicate solution, which is called a waterglass, is used as a starting material has been known (JP-A-61-158810).In this production method, a sodium silicate solution is once treatedwith a cation exchange resin to remove ions such as sodium ions, therebyincreasing the purity as a starting material, and then used for theproduction of a silica sol.

However, according to the production method described in JP-A-61-158810,the increase in purity of the starting material by ion exchange islimited.

Thus, as a method for obtaining a high-purity silica sol, a method thatutilizes the hydrolysis of a high-purity alkoxysilane, such astetraethyl ortho silicate, has been disclosed. As such a method, WO2017/022552 (corresponding to U.S. Patent Application Publication No.2019/010059) discloses a method for producing a silica sol, including astep of preparing a reaction liquid by mixing a liquid (B) containing analkoxysilane or a condensate thereof and a second organic solvent and aliquid (C) containing water with a liquid (A) containing an alkalicatalyst, water, and a first organic solvent. Then, it is disclosed thatthis production method allows for the stable production of a silica solin which the particle size of silica particles is uniform.

SUMMARY

In recent years, there is an increasing demand for an increase in theamount of silica that can be produced per batch, thereby achievingfurther improved productivity. Thus, the present inventors have foundthat in the conventional sol-gel method for producing a silica sol, whenthe amount of raw materials added is increased in order to increase theamount of silica, fine particles are formed, and the homogeneity of thesilica sol decreases, possibly resulting in a problem in that sufficientquality cannot be obtained.

Thus, an object of the present invention is to provide a means capableof suppressing the formation of fine particles in a method for producinga silica sol.

The above object of the present invention can be achieved by thefollowing means.

A method for producing a silica sol, including synthesizing a silica solby, in a reaction liquid containing an alkoxysilane or a condensatethereof, water, and an alkali catalyst, allowing the alkoxysilane orcondensate thereof to react with the water in the presence of the alkalicatalyst, wherein

the alkali catalyst is not additionally supplied after the start of thesynthesis until the finish time of the synthesis, and

during 90% or more of the time between when 5 minutes have elapsed fromthe time point when the electrical conductivity of the reaction liquidreaches a local maximum for the first time from the start of thereaction and the finish time of the synthesis, the proportion of thevalue of the electrical conductivity of the reaction liquid is more than90% relative to the value of the electrical conductivity at the timewhen 5 minutes have elapsed from the time point when the local maximumis reached.

As used herein, “the alkali catalyst is not additionally supplied afterthe start of the synthesis until the finish time of the synthesis” meansthat substantially no alkali catalyst is additionally supplied duringthis period. “Substantially no alkali catalyst is additionally supplied”means that the addition of the alkali catalyst itself, or, as is clearfrom the below description, the addition of a liquid containing analkali catalyst at a concentration of more than 1 ppm based on the totalmass or a liquid containing an alkali catalyst having a pH of 8.0 ormore is not intentionally performed. Among them, it is preferable thatcompletely no alkali catalyst is additionally supplied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a reaction liquid temperature-reaction time graph in theproduction method according to Example 1;

FIG. 1B is a reaction liquid temperature-reaction time graph in theproduction method according to Comparative Example 1;

FIG. 2A is an electrical conductivity-reaction time graph in theproduction method according to Example 1;

FIG. 2B is an electrical conductivity-reaction time graph in theproduction method according to Comparative Example 1;

FIG. 3A is an SEM image of silica particles contained in a silica solproduced by the production method according to Example 1. Here, 2indicates silica particles that are not fine particles (main particles);and

FIG. 3B is an SEM image of silica particles contained in a silica solproduced by the production method according to Comparative Example 1.Here, 1 indicates silica fine particles, and 2 indicates silicaparticles that are not fine particles (main particles).

DETAILED DESCRIPTION

Hereinafter, the present invention will be described. Incidentally, thepresent invention is not limited only to the following embodiments. Asused herein, “X to Y” showing a range means “X or more and Y or less”.In addition, as used herein, unless otherwise noted, the operations andthe measurement of physical properties and the like are performed underconditions of room temperature (20 to 25° C.)/relative humidity of 40 to50% RH.

Hereinafter, embodiments of the present invention will be described withreference to the attached drawings. Incidentally, in the description ofdrawings, same elements will be indicated with same symbols, andredundant description will be omitted.

<Method for Producing Silica Sol>

One aspect of the present invention relates to a method for producing asilica sol, the method including synthesizing a silica sol by, in areaction liquid containing an alkoxysilane or a condensate thereof,water, and an alkali catalyst, allowing the alkoxysilane or condensatethereof to react with the water in the presence of the alkali catalyst.After the start of the synthesis until the finish time of the synthesis,the alkali catalyst is not additionally supplied, and, during 90% ormore of the time between when 5 minutes have elapsed (hereinafter alsosimply referred to as “when 5 minutes have elapsed from the first localmaximum”) from the time point when the electrical conductivity of thereaction liquid reaches a local maximum for the first time from thestart of reaction (hereinafter also simply referred to as “firstlocal-maximum time point”) and the finish time of the synthesis, theproportion of the value of the electrical conductivity is more than 90%relative to the value of the electrical conductivity at the time when 5minutes have elapsed from the time point when the local maximum isreached. According to the present invention, a means capable ofsuppressing the formation of fine particles in a method for producing asilica sol can be provided.

With respect to the mechanism that such a configuration can suppress theformation of fine particles in a method for producing a silica sol, thepresent inventors surmise as follows.

In the method for producing a silica sol according to one embodiment ofthe present invention, for example, in the case where tetramethoxysilaneis used as the raw material alkoxysilane or condensate thereof, thechemical reaction that generates a silica sol (synthesis reaction) isexpressed as in the following reaction formula (1).

In the generation of a silica sol, the reaction rate is determined bytetramethoxysilane (Si(OCH₃)₄) as a starting material, water (H₂O) forhydrolysis, and an alkali catalyst as a catalyst.

An alkali catalyst dissociates itself in water or dissociates water, andyields hydroxide ions. For example, in the case where ammonia (NH₃) isused as an alkali catalyst, ammonia shows equilibrium as represented bythe following formula in water.[Chemical Formula 2]NH₃+H₂O

NH₄ ⁺+OH⁻  Reaction formula (2)

In a method for producing a silica sol, including allowingtetramethoxysilane to react with water in the presence of ammonia, whichis an alkali catalyst, as in the production method according to WO2017/022552 (corresponding to U.S. Patent Application Publication No.2019/010059), generally, a liquid containing tetramethoxysilane is addedto a liquid containing ammonia, thereby mixing the two. In addition, inthe conventional silica sol production, such as the production methodaccording to WO 2017/022552 (corresponding to U.S. Patent ApplicationPublication No. 2019/010059, Specification), when the amount oftetramethoxysilane-containing liquid added increases, with an increasein the total mass of the reaction liquid, the ammonia content relativeto the total mass decreases, resulting in a decrease in theconcentration of ammonia in the reaction liquid. Then, when theconcentration of ammonia falls below a certain level, the frequency offormation of fine particles increases, and the homogeneity of the silicasol decreases. The reason for this is presumably as follows. Hydroxideions serve to adhere the generated silica to each other. Accordingly,when the concentration of ammonia decreases in the final stage of thereaction, and the concentration of hydroxide ions also decreases, silicaparticles cannot grow sufficiently, resulting in the formation of fineparticles.

Here, the concentration of ions in a reaction liquid and the electricalconductivity of the reaction liquid are correlated. Therefore, after thestart of the reaction, in the state where the reaction is stablyproceeding, when the rate of decrease in electrical conductivity is madeto fall below a predetermined value, and the electrical conductivity ismaintained at or higher than a predetermined proportion, the decrease inconcentration of hydroxide ions also falls below a predetermined value.As a result, even in the final stage of the reaction, the silicaadhesion effect of hydroxide ions can be maintained, and silicaparticles can grow sufficiently, whereby the formation of fine particlescan be suppressed.

Incidentally, the above mechanism is based on supposition, and thepresent invention is not limited to the above mechanism by any means.Hereinafter, the configuration of the production method according to thepresent invention will be described in detail.

The production method according to one embodiment of the presentinvention includes synthesizing a silica sol by allowing an alkoxysilaneor a condensate thereof to react with water in the presence of an alkalicatalyst in a reaction liquid containing an alkoxysilane or a condensatethereof, water, and an alkali catalyst.

As such a silica sol synthesis method, any of known methods can be usedwithout particularly limitations. Among them, methods of two-componentreaction type and three-component reaction type are known. In thetwo-component reaction type, a liquid containing an alkoxysilane or acondensate thereof and an organic solvent (addition side) is added to aliquid containing an alkali catalyst, water, and an organic solvent(receiver side). In the two-component reaction type, the liquidcontaining an alkali catalyst, water, and an organic solvent on thereceiver side contains all of water that serves as one of the componentsthat determine the reaction rate. Meanwhile, in the three-componentreaction type, a liquid containing an alkoxysilane or a condensatethereof and an organic solvent (addition side) and a liquid containingwater (addition side) are added to a liquid containing an alkalicatalyst, water, and an organic solvent (receiver side). Among them, itis more preferable that the production method according to oneembodiment of the present invention is applied to a synthesis method ofthree-component reaction type. According to a synthesis method ofthree-component reaction type, the suppressing effect on the formationof fine particles can be further enhanced.

Incidentally, as used herein, in the case of a method for producing asilica sol including adding an addition-side liquid to a receiver-sideliquid, such as a method of two-component reaction type orthree-component reaction type, “start of the synthesis” means that theaddition is started. In addition, in this case, “start time of thesynthesis” means the time point when the addition is started, and “afterthe start of the synthesis” means the time from immediately after thestart of the addition. Then, in this case, “finish of the synthesis”means that the addition is finished, and “finish time of the synthesis”means the time point when the addition is finished.

The synthesis method of three-component reaction type is notparticularly limited, and a known method can be used. However, it ispreferable to use a synthesis method including preparing a reactionliquid by mixing a liquid (B) containing an alkoxysilane or a condensatethereof and a second organic solvent (herein also referred to as “liquid(B)”) and a liquid (C) containing water (herein also referred to as“liquid (C)”) with a liquid (A) containing an alkali catalyst, water,and a first organic solvent (herein also referred to as “liquid (A)”).In the reaction liquid, the alkoxysilane or condensate thereof undergoeshydrolysis and polycondensation, whereby a silica sol is generated.

[Liquid (A) Containing Alkali Catalyst, Water, and First OrganicSolvent]

In the method for producing a silica sol according to one embodiment ofthe present invention, the liquid (A) containing an alkali catalyst,water, and a first organic solvent for use in the synthesis of a silicasol can be prepared by mixing an alkali catalyst, water, and a firstorganic solvent. In addition to the alkali catalyst, water, and organicsolvent, the liquid (A) can also contain other components withoutinterfering with the effects of the present invention.

As the alkali catalyst contained in the liquid (A), any of thoseconventionally known can be used. For the reason that the contaminationwith metal impurities and the like can be minimized, it is preferablethat the alkali catalyst is at least one of ammonia, tetramethylammonium hydroxide, and other ammonium salts. Among them, in terms ofthe excellent catalytic action, ammonia is more preferable. Ammonia hashigh volatility and thus can be easily removed from the silica sol.Incidentally, the alkali catalyst may be used alone, and it is alsopossible to use a combination of two or more kinds.

As water contained in the liquid (A), in terms of minimizing thecontamination with metal impurities and the like, it is preferable touse pure water or ultrapure water.

As the first organic solvent contained in the liquid (A), it ispreferable to use a hydrophilic organic solvent. Specific examplesthereof include alcohols such as methanol, ethanol, n-propanol,isopropanol, ethylene glycol, propylene glycol, and 1,4-butanediol;ketones such as acetone and methyl ethyl ketone; and the like. Amongthem, an alcohol is preferable as the first organic solvent. Use of analcohol is effective in that when the silica sol is subjected to thebelow-described water displacement, the alcohol and water can be easilydisplaced by heating distillation. In addition, in terms of the recoveryor recycling of organic solvents, it is preferable to use the same kindof alcohol as the alcohol resulting from the hydrolysis of thealkoxysilane. Among alcohols, in particular, it is more preferable touse at least one kind selected from the group consisting of methanol,ethanol, and isopropanol. In the case where tetramethoxysilane is usedas the alkoxysilane, it is preferable that the first organic solvent ismethanol. The first organic solvent may be used alone, and it is alsopossible to use a combination of two or more kinds.

The combination of an alkali catalyst, water, and a first organiccatalyst in the liquid (A) is not particularly limited. The kind of eachcan be suitably changed in order to obtain particles of the desired kindand also particles having the desired characteristics, particle size,particle size distribution, and the like, and their contents can also besuitably adjusted.

In the production method according to one embodiment of the presentinvention, the amount of fine particles can be controlled by controllingthe content of the alkali catalyst in the liquid (A). The lower limit onthe content of the alkali catalyst in the liquid (A) is not particularlylimited, but is in terms of further promoting the action as a hydrolysiscatalyst or grain growth, that is, further enhancing the suppressingeffect on the formation of fine particles, preferably 0.1 mass % ormore, more preferably 0.3 mass % or more, and still more preferably 0.5mass % or more based on the total mass of the liquid (A) (100 mass %).In addition, the upper limit on the content of the alkali catalyst inthe liquid (A) is not particularly limited, but is, in terms ofproductivity and cost, preferably 50 mass % or less, more preferably 40mass % or less, still more preferably 20 mass % or less, andparticularly preferably 10 mass % or less.

The lower limit on the water content in the liquid (A) is adjustedaccording to the amount of alkoxysilane or condensate thereof used forthe reaction and is not particularly limited, but is, in terms of thehydrolysis of the alkoxysilane, preferably 1 mass % or more, morepreferably 5 mass % or more, and still more preferably 10 mass % or morebased on the total mass of the liquid (A) (100 mass %). In addition, theupper limit on the water content in the liquid (A) is not particularlylimited, but is, in terms of miscibility with the liquid (B), preferably50 mass % or less, more preferably 40 mass % or less, and still morepreferably 30 mass % or less based on the total mass of the liquid (A)(100 mass %).

The lower limit on the content of the first organic solvent is notparticularly limited, but is, in terms of miscibility with the liquid(B), preferably 10 mass % or more, more preferably 20 mass % or more,and still more preferably 50 mass % or more based on the total mass ofthe liquid (A) (100 mass %). In addition, the upper limit on the contentof the first organic solvent is not particularly limited, but is, interms of dispersibility, preferably 98 mass % or less, more preferably95 mass % or less, and still more preferably 90 mass % or less based onthe total mass of the liquid (A) (100 mass %).

[Liquid (B) Containing Alkoxysilane or Condensate Thereof and SecondOrganic Solvent]

In the method for producing a silica sol according to one embodiment ofthe present invention, the liquid (B) containing an alkoxysilane or acondensate thereof and a second organic solvent for use in the synthesisof a silica sol can be prepared by mixing an alkoxysilane or acondensate thereof with a second organic solvent. When an alkoxysilaneor a condensate thereof is dissolved in an organic solvent and thenmixed with the liquid (A), the silica sol synthesis reaction proceedsmore mildly, whereby the formation of a gel-like product can be furthersuppressed, and the miscibility can also further improve. In addition tothe alkoxysilane or condensate thereof and second organic solvent, theliquid (B) can also contain other components without interfering withthe effects of the present invention. However, it is preferable that theliquid (B) contains substantially no alkali catalyst, and it isparticularly preferable that an alkali catalyst is not contained at all,that is, its content is 0 mass % based on the total mass of the liquid(B). When the liquid (B) contains substantially no alkali catalyst, alack of uniformity in the alkali catalyst concentration in the reactionliquid can be suppressed, and the suppressing effect on the formation offine particles can be enhanced. Incidentally, as used herein,“containing substantially no alkali catalyst” means that the content is1 ppm or less based on the total mass of the liquid. Here, when thecontent of the alkali catalyst is 1 ppm or less based on the total massof the liquid, such an alkali catalyst is treated as unintentionallyincorporated impurities, and even when such a liquid is added after thestart of the synthesis until the finish time of the synthesis, suchaddition is not regarded as additional supply of an alkali catalyst.

In addition, it is preferable that the liquid (B) contains substantiallyno water, and it is particularly preferable that water is not containedat all, that is, its content is 0 mass % based on the total mass of theliquid (B). Incidentally, as used herein, “containing substantially nowater” means that the content is 0.1 mass % or less based on the totalmass of the liquid.

As the alkoxysilane or condensate thereof contained in the liquid (B),any of known ones can be used without particularly limitations. Examplesthereof include tetramethoxysilane, tetraethoxysilane,tetrapropoxysilane, condensates thereof, and the like. Among them, interms of having appropriate hydrolysis reactivity, tetramethoxysilane ispreferable. In addition, the alkoxysilane or condensate thereof may beused alone, and it is also possible to use a combination of two or morekinds.

As the second organic solvent contained in the liquid (B), it ispreferable to use a hydrophilic organic solvent. Specific examplesthereof include alcohols such as methanol, ethanol, n-propanol,isopropanol, ethylene glycol, propylene glycol, and 1,4-butanediol;ketones such as acetone and methyl ethyl ketone; and the like. Amongthem, an alcohol is preferable as the second organic solvent. As aresult of using an alcohol, when the silica sol obtained by thesynthesis is subjected to the below-described water displacement, thealcohol and water can be easily displaced by heating distillation. Inaddition, as the second organic solvent, in terms of the recovery orrecycling of organic solvents, it is preferable to use the same kind ofalcohol as the alcohol resulting from the hydrolysis of thealkoxysilane. Among alcohols, methanol, ethanol, isopropanol, and thelike are more preferable. For example, in the case wheretetramethoxysilane is used as the alkoxysilane, it is preferable thatthe second organic solvent is methanol. Further, in terms of therecovery or recycling of organic solvents, it is preferable that thesecond organic solvent is the same as the first organic solventcontained in the liquid (A). Incidentally, the second organic solventmay be used alone, and it is also possible to use a combination of twoor more kinds.

With respect to the combination of an alkoxysilane or a condensatethereof and a second organic solvent in the liquid (B), the kind of eachcan be suitably changed in order to obtain particles of the desired kindand also particles having the desired characteristics, particle size,particle size distribution, and the like, and their contents can also besuitably adjusted.

In the production method according to one embodiment of the presentinvention, the lower limit on the content of the alkoxysilane orcondensate thereof in the liquid (B) is not particularly limited, butis, in terms of further enhancing the concentration of silica in thereaction liquid and further improving the productivity, preferably 50mass % or more, more preferably 60 mass % or more, and still morepreferably 70 mass % or more based on the total amount of the liquid (B)(100 mass %). In addition, the upper limit on the alkoxysilane orcondensate thereof in the liquid (B) is not particularly limited, butis, in terms of allowing the silica sol synthesis reaction to proceedmore mildly, thereby further suppressing the formation of a gel-likeproduct, and also in terms of miscibility, preferably 98 mass % or less,more preferably 95 mass % or less, and still more preferably 90 mass %or less based on the total mass of the liquid (B) (100 mass %).

In addition, the lower limit on the content of the second organicsolvent in the liquid (B) is not particularly limited, but is, in termsof allowing the silica sol synthesis reaction to proceed more mildly,thereby further suppressing the formation of a gel-like product, andalso in terms of miscibility, preferably 2 mass % or more, morepreferably 5 mass % or more, and still more preferably 10 mass % or morebased on the total mass of the liquid (B) (100 mass %). In addition, theupper limit on the content of the second organic solvent in the liquid(B) is not particularly limited, but is, in terms of further enhancingthe concentration of silica in the reaction liquid and further improvingthe productivity, preferably 50 mass % or less, more preferably 40 mass% or less, and still more preferably 30 mass % or less.

[Liquid (C) Containing Water]

In the method for producing a silica sol according to one embodiment ofthe present invention, in terms of minimizing the contamination withmetal impurities and the like, it is preferable that water in the liquid(C) containing water for use in the synthesis of a silica sol is purewater or ultrapure water. In addition, the liquid (C) can also containother components in addition to water without interfering with theeffects of the present invention. However, it is preferable that theliquid (C) contains substantially no alkali catalyst, and it isparticularly preferable that an alkali catalyst is not contained at all,that is, its content is 0 mass % based on the total mass of the liquid(C). When the liquid (C) contains substantially no alkali catalyst, alack of uniformity in the alkali catalyst concentration in the reactionliquid can be suppressed, and the suppressing effect on the formation offine particles can be enhanced. Incidentally, as used herein,“containing substantially no alkali catalyst” means that the content is1 ppm or less based on the total mass of the liquid. Here, when thecontent of the alkali catalyst is 1 ppm or less based on the total massof the liquid, such an alkali catalyst is treated as unintentionallyincorporated impurities, and even when such a liquid is added after thestart of the synthesis until the finish time of the synthesis, suchaddition is not regarded as additional supply of an alkali catalyst.

The lower limit on the pH value of the liquid (C) is not particularlylimited, but is, in terms of further suppressing the gelation of thereaction liquid, preferably 5.0 or more, more preferably 5.5 or more,still more preferably 6.0 or more, and particularly preferably 6.5 ormore. In addition, the upper limit on the pH value of the liquid (C) isnot particularly limited, but is preferably less than 8.0. Within thisrange, a lack of uniformity in the concentration of hydroxide ions inthe reaction liquid is further suppressed, and the suppressing effect onthe formation of fine particles can be further enhanced. Here, the pHcan be measured with a desktop pH meter (Model No.: F-72) manufacturedby HORIBA, Ltd., for example.

From above, as preferred examples of the liquid (C), a liquid (C1)containing water and having a pH of 5.0 or more and less than 8.0, aliquid (C2) containing water and substantially no alkali catalyst, andthe like can be mentioned. Then, a liquid (C3) containing water andsubstantially no alkali catalyst and having a pH of 5.0 or more and lessthan 8.0 is more preferable.

Therefore, as an example of the method for producing a silica solaccording to one embodiment of the present invention, it is preferableto include preparing a reaction liquid by mixing a liquid (B) containingan alkoxysilane or a condensate thereof and a second organic solvent anda liquid (C1) containing water and having a pH of 5.0 or more and lessthan 8.0 with a liquid (A) containing an alkali catalyst, water, and afirst organic solvent.

Incidentally, as used herein, even if a liquid contains an alkalicatalyst, when its pH is less than 8.0, such an alkali catalyst istreated as unintentionally incorporated impurities, and, even when sucha liquid is added after the start of the synthesis until the finish timeof the synthesis, such addition is not regarded as additional supply ofan alkali catalyst.

In addition, as another example of the method for producing a silica solaccording to one embodiment of the present invention, it is preferableto include a step of preparing a reaction liquid by mixing a liquid (B)containing an alkoxysilane or a condensate thereof and a second organicsolvent and a liquid (C2) containing water and not containing the alkalicatalyst with a liquid (A) containing an alkali catalyst, water, and afirst organic solvent.

[Preparation of Reaction Liquid and Synthesis of Silica Gel]

The production method according to one embodiment of the presentinvention preferably includes preparing a reaction liquid by mixing analkoxysilane or a condensate thereof, water, and an alkali catalyst.Here, in the case where a synthesis method of two-component reactiontype is employed, a method for preparing the reaction liquid is notparticularly limited, and may be, for example, a method in which aliquid containing an alkoxysilane or a condensate thereof and an organicsolvent and a liquid containing an alkali catalyst, water, and anorganic solvent are mixed, for example. In addition, in the case where asynthesis method of three-component reaction type is employed, a methodfor preparing the reaction liquid is not particularly limited, and maybe, for example, a method in which the above liquid (B) and the aboveliquid (C) are mixed with the above liquid (A) to prepare a reactionliquid, for example.

In the production method according to one embodiment of the presentinvention, a method for mixing two components in the two-componentreaction type or three components in the three-component reaction typeis not particularly limited, but it is preferable that the alkoxysilaneor condensate thereof is added at a constant addition rate from thestart time of the synthesis of a silica sol until the finish time of thesynthesis.

Incidentally, as used herein, “constant addition rate” means that thevariation width of the rate of addition of the additive liquid is withina range of ±30% or less relative to the average addition rate calculatedby dividing the total amount of the addition-side liquid by the timefrom the start time of the synthesis until the finish time of thesynthesis (total mass (g) of the addition-side liquid÷time from thestart time of the synthesis until the finish time of the synthesis(min)). For example, in the case of a method of three-component reactiontype, it means that the width is within a range of ±30% or less relativeto each of the average addition rates of the liquid (B) and the liquid(C) each calculated by dividing the total mass (g) of the liquid (B) orthe liquid (C) added to the liquid (A) by the time from the time pointwhen the addition is started until the time point when the addition isfinished (total mass (g) of the liquid (B) or liquid (C) added÷time fromthe time point when the addition is started until the time point whenthe addition is finished (min)).

The alkoxysilane or condensate thereof undergoes hydrolysis andpolycondensation in such a reaction liquid, whereby a silica sol isgenerated. The silica sol may be used as it is according to the intendeduse, or alternatively, may also be subjected to the below-describedwater displacement step or concentration step and used as the resultingliquid, or dispersed in an organic solvent and used as an organosol.

In the production method according to one embodiment of the presentinvention, an alkali catalyst is not additionally supplied after thestart of the synthesis of a silica sol until the finish time of thesynthesis. Therefore, in the case where a synthesis method ofthree-component reaction type is employed, the alkali catalyst ispresent only in the liquid (A), and no alkali catalyst is additionallysupplied later.

In the method for producing a silica sol according to one embodiment ofthe present invention, without additionally supplying an alkalicatalyst, the formation of fine particles can be significantlysuppressed. The reason for this is unknown in detail, but is presumablyas follows. As described above, after the start of the reaction, in thestate where the reaction is stably proceeding, when the rate of decreasein the electrical conductivity is made to fall below a predeterminedvalue, the concentration of hydroxide ions in the reaction liquidbecomes approximately constant even without additionally supplyingammonia. As a result, even in the final stage of the reaction, thesilica adhesion effect of hydroxide ions can be maintained, and silicaparticles can grow sufficiently, whereby the formation of fine particlescan be suppressed.

In the case where a synthesis method of three-component reaction type isemployed, the method for adding the liquid (B) and the liquid (C) at thetime of mixing the liquid (B) and the liquid (C) with the liquid (A) isnot particularly limited. Approximately constant amounts of the liquidsmay be simultaneously added to the liquid (A), or it is also possiblethat the liquid (B) and the liquid (C) are alternately added to theliquid (A). Alternatively, the liquid (B) and the liquid (C) may also beadded at random. Among them, in terms of suppressing changes in theamount of water in the reaction liquid used for the synthesis reaction,it is preferable to use a method in which the liquid (B) and the liquid(C) are simultaneously added, and it is more preferable to use a methodin which the liquid (B) and the liquid (C) are simultaneously added eachat a constant addition rate.

As a method for adding the liquid (B) and the liquid (C) to the liquid(A), in terms of further suppressing a lack of uniformity in the alkalicatalyst concentration in the reaction liquid, and further enhancing thesuppressing effect on the formation of fine particles, it is preferablethat the liquid (B) and the liquid (C) are added to the liquid (A) bydivided addition or continuous addition. Here, divided addition meansthat when the liquid (B) and the liquid (C) are added to the liquid (A),the whole amount of the liquid (B) and the liquid (C) is added not atonce, but is added in two or more portions discontinuously orcontinuously. As a specific example of divided addition, dropwiseaddition can be mentioned. In addition, continuous addition means thatwhen the liquid (B) and the liquid (C) are added to the liquid (A), thewhole amount of the liquid (B) and the liquid (C) is added not at once,but is added continuously without interrupting the addition.

The time required for adding the whole amount of the liquid (B) and theliquid (C) to the liquid (A) changes depending on the liquid amounts ofthe liquid (B) and the liquid (C) and thus is not particularly limited,but is preferably 10 minutes or more, more preferably 15 minutes ormore, and still more preferably 20 minutes or more. Within this range, alack of uniformity in the alkali catalyst concentration in the reactionliquid is further suppressed, and the suppressing effect on theformation of fine particles can be further enhanced. In addition, theupper limit on the time required for adding the whole amount of theliquid (B) and the liquid (C) to the liquid (A) is not particularlylimited, but is preferably 300 minutes or less in terms of productivity.

From this, as a preferred method for adding the liquid (B) and theliquid (C) at the time of mixing the liquid (B) and the liquid (C) withthe liquid (A), in terms of further suppressing a lack of uniformity inthe alkali catalyst concentration in the reaction liquid, and furtherenhancing the suppressing effect on the formation of fine particles, amethod in which the liquid (B) and the liquid (C) are simultaneouslyadded each at a constant addition rate, and the addition is completedwithin a certain period of time or longer.

The lower limits on the temperatures of the liquid (A), the liquid (B),and the liquid (C) are not particularly limited, but are eachindependently preferably 0° C. or more, more preferably 10° C. or more,and still more preferably 20° C. or more. In addition, the upper limitson the temperatures of the liquid (A), the liquid (B), and the liquid(C) are each independently preferably 70° C. or less, more preferably60° C. or less, and still more preferably 50° C. or less. That is, it ispreferable that the temperatures of the liquid (A), the liquid (B), andthe liquid (C) are each independently 0° C. or more and 70° C. or less.When the temperature is 0° C. or more, each liquid (liquid (A), liquid(B), liquid (C)) can be prevented from freezing. Meanwhile, when thetemperature is 70° C. or less, the volatilization of organic solventscan be prevented.

In addition, the difference in temperature among the liquid (A), theliquid (B), and the liquid (C) is preferably not more than 20° C., morepreferably not more than 10° C., and still more preferably 0° C. (lowerlimit: 0° C.). Here, the difference in temperature means the differencebetween the highest and lowest temperatures among the three liquids.

As used herein, in the case where a synthesis method of three-componentreaction type using the liquid (A), the liquid (B) and the liquid (C) isemployed, “reaction liquid” is a liquid obtained by mixing the liquid(B) and the liquid (C) with the liquid (A), and means a liquid in thestate where the hydrolysis and polycondensation of the alkoxysilane orcondensate thereof are about to proceed (before proceeding).

In the method for producing a silica sol according to one embodiment ofthe present invention, during 90% or more of the time between when 5minutes have elapsed (when 5 minutes have elapsed from the first localmaximum) from the time point when the electrical conductivity of thereaction liquid reaches a local maximum for the first time from thestart of the reaction (first local-maximum time point) and the finishtime of the synthesis of a silica sol, the proportion of the value ofthe electrical conductivity of the reaction liquid is more than 90%relative to the value of the electrical conductivity at the time when 5minutes have elapsed from the time point when the local maximum isreached.

In the case where the proportion of the value of the electricalconductivity of the reaction liquid during the above period is 90% orless relative to the value of the electrical conductivity at the timewhen 5 minutes have elapsed from the first local maximum, the frequencyof formation of fine particles significantly increases. In addition, interms of further enhancing the suppressing effect on the formation offine particles, the proportion of the value of the electricalconductivity of the reaction liquid during the above period relative tothe value of the electrical conductivity at the time when 5 minutes haveelapsed from the first local maximum is preferably more than 95%, andmore preferably 100% or more. In addition, in terms of suppressing rapidgrowth of silica particles, the proportion of the value of theelectrical conductivity of the reaction liquid during the above periodrelative to the value of the electrical conductivity at the time when 5minutes have elapsed from the first local maximum is preferably 200% orless.

The reason why the formation of fine particles is significantlysuppressed by making the rate of decrease in the electrical conductivityof the reaction liquid during the silica sol synthesis fall below apredetermined value is unknown in detail, but is presumably as follows.As described above, the electrical conductivity of the reaction liquidis correlated to the ion concentration in the reaction liquid, and whenthe rate of decrease in the electrical conductivity is made to fallbelow a predetermined value, and the electrical conductivity ismaintained at or higher than a predetermined proportion, the rate ofdecrease in the concentration of hydroxide ions also falls below apredetermined value. As a result, even in the final stage of thereaction, the silica adhesion effect of hydroxide ions can bemaintained, and silica particles can grow sufficiently, whereby theformation of fine particles can be suppressed.

In addition, in the case where the proportion of the value of theelectrical conductivity of the reaction liquid during the above periodis 90% or less relative to the value of the electrical conductivity atthe time when 5 minutes have elapsed from the first local maximum, whenit is attempted to reduce the frequency of formation of fine particleseven a little, it may be necessary to reduce the amount of liquidcontaining the alkoxysilane or condensate thereof (for example,tetramethoxysilane) added, and, in this case, the amount of silicaparticles synthesized per batch also decreases. However, according tothe method for producing a silica sol according to one embodiment of thepresent invention, because the formation of fine particles issignificantly suppressed, the amount of liquid containing thealkoxysilane or condensate thereof (for example, tetramethoxysilane)added can be increased, and the amount of silica particles synthesizedper batch can be increased. This is also effective in that theproductivity can further enhanced.

Here, the electrical conductivity (μS/cm) of the reaction liquid can bemeasured using Lacom Tester pH & Conductivity Meter PCWP300 manufacturedby Universal Technics Co., Ltd., or the like, for example. Incidentally,the details of the measurement method will be described in the Examples.

In addition, when the time during which the proportion of the value ofthe electrical conductivity of the reaction liquid is more than 90%relative to the value of the electrical conductivity at the time when 5minutes have elapsed from the first local maximum is less than 90%relative to the time between when 5 minutes have elapsed from the firstlocal maximum to the finish time of the synthesis of a silica sol, thefrequency of formation of fine particles significantly increases. Interms of further enhancing the suppressing effect on the formation offine particles, the time is 90% or more, preferably 95% or more, andmore preferably 100% (upper limit: 100%).

Relative to the total amount of the alkoxysilane or condensate thereofadded from the start time of the synthesis of a silica sol until thefinish time of the synthesis, the proportion (%) of the amount ofalkoxysilane or condensate thereof added from the start time of thesynthesis until the first local-maximum time point is preferably lessthan 20 mass %, more preferably 15 mass % or less, and still morepreferably 10 mass % or less (lower limit: more than 0 mass %). Withinthis range, the suppressing effect on the formation of fine particles isfurther enhanced. Here, in the case where the alkoxysilane or condensatethereof is added at a constant addition rate from the start time of thesynthesis of a silica sol until the finish time of the synthesis, thefollowing can be said. For example, in the case where a synthesis methodof three-component reaction type is employed, and the liquid (B) isadded to the liquid (A) at a constant addition rate, the aboveproportion is equal to the proportion of the time from the start time ofthe synthesis of a silica sol until the first local-maximum time pointrelative to the time from the start time of the synthesis of a silicasol (start time of the addition) until the finish time of the synthesis(finish time of the addition).

In addition, relative to the total amount of the alkoxysilane orcondensate thereof added between when 5 minutes have elapsed from thefirst local maximum and the finish time of the synthesis, the proportion(%) of the amount of the alkoxysilane or condensate thereof added duringthis period in the state where the proportion of the value of theelectrical conductivity of the reaction liquid is more than 90% relativeto the value of the electrical conductivity at the time when 5 minuteshave elapsed from the first local maximum is preferably 90 mass % ormore, more preferably 95 mass % or more, and still more preferably 100mass % (upper limit: 100 mass %). Within this range, the suppressingeffect on the formation of fine particles is further enhanced. Here, inthe case where the alkoxysilane or condensate thereof is added at aconstant addition rate from the start time of the synthesis of a silicasol until the finish time of the synthesis (or at least from when 5minutes have elapsed from the first local maximum until the finish timeof the synthesis of a silica sol), the following can be said. Forexample, in the case where a synthesis method of three-componentreaction type is employed, and the liquid (B) is added to the liquid (A)at a constant addition rate, the above proportion is equal to theproportion (%) of, relative to the time from when 5 minutes have elapsedfrom the first local maximum until the finish time of the synthesis of asilica sol, the time during which the state where the proportion of thevalue of the electrical conductivity of the reaction liquid is more than90% relative to the value at the time when 5 minutes have elapsed fromthe first local maximum is presented within this time.

The temperature of the reaction liquid is not particularly limited aslong as the above electrical conductivity relationship is satisfied.Preferred values of the upper and lower limits of the temperature of thereaction liquid are the same as the preferred values of the upper andlower limits of the temperatures of the above liquid (A), the liquid(B), and the liquid (C), respectively.

The method for producing a silica sol according to one embodiment of thepresent invention preferably includes lowering the temperature of thereaction liquid stepwise or continuously and more preferably includeslowering the temperature of the reaction liquid continuously, during atleast part of the time between the start time of the synthesis of asilica sol and the finish time of the synthesis. Incidentally, as usedherein, “the temperature of a reaction liquid is lowered continuously”means that when the temperature is measured every 5 minutes, atemperature that is 0.2° C. or more lower than the temperature at theprevious measurement time is confirmed.

In addition, in the production method, between when 5 minutes haveelapsed from the first local maximum and the finish time of thesynthesis, the time for continuously lowering the temperature of thereaction liquid is preferably 50% or more, more preferably 80% or more,of the time. Then, it is still more preferable that the method includescontinuously lowering the temperature of the reaction liquid during theentire time between when 5 minutes have elapsed from the first localmaximum and the finish time of the synthesis. By lowering thetemperature of the reaction liquid, the suppressing effect on theformation of fine particles can be further enhanced.

When the temperature of the reaction liquid is lowered, the electricalconductivity of the reaction liquid increases. Therefore, it ispreferable that the temperature of the reaction liquid is adjusted suchthat from when 5 minutes have elapsed from the first local maximum untilthe finish time of the synthesis of a silica sol, the electricalconductivity of the reaction liquid is more than 90% relative to thevalue of the electrical conductivity at the time when 5 minutes haveelapsed from the first local maximum.

The reason why the formation of fine particles is significantlysuppressed by lowering the temperature of the reaction liquid is unknownin detail, but is presumably as follows. When the temperature of thereaction liquid is lowered, the equilibrium of the above reactionformula (2) inclines to the right, the concentration of hydroxide ionsincreases, and the electrical conductivity of the reaction liquidincreases. Then, because of an increase in the hydroxide ionconcentration, even in the final stage of the reaction, the silicaadhesion effect of hydroxide ions can be maintained, and silicaparticles can grow sufficiently, whereby the suppressing effect on theformation of fine particles is further enhanced.

The temperature of the reaction liquid can be measured using LacomTester pH & Conductivity Meter PCWP300 manufactured by UniversalTechnics Co., Ltd., or the like, for example.

In the method for producing a silica sol according to one embodiment ofthe present invention, the hydrolysis and the polycondensation reactioncan be performed under any of the following pressure conditions: reducedpressure, atmospheric pressure, and increased pressure. However, interms of production cost, performance under atmospheric pressure ispreferable.

The molar ratio of the alkoxysilane or condensate thereof, water, thealkali catalyst, and the first and second organic solvents in thereaction liquid is not particularly limited, and can be suitablyadjusted with the contents of the alkali catalyst contained in theliquid (A) or the alkoxysilane or condensate thereof contained in theliquid (B).

As described above, as used herein, in the case where a synthesis methodof three-component reaction type using the liquid (A), the liquid (B)and the liquid (C) is employed, “reaction liquid” is a liquid obtainedby mixing the liquid (B) and the liquid (C) with the liquid (A), andmeans a liquid in the state where the hydrolysis and polycondensation ofthe alkoxysilane or condensate thereof are about to proceed (beforeproceeding). Meanwhile, “silica sol” means a liquid in which hydrolysisand polycondensation have been finished.

That is, the molar ratio of water, the alkali catalyst, and the organicsolvents (the total amount of the first and second organic solvents) isthe molar ratio of the alkoxysilane or condensate thereof, water, thealkali catalyst, and the organic solvents (the total amount of the firstand second organic solvents) contained in the whole amount of thereaction liquid when all the liquid (A), the liquid (B), and the liquid(C) used for the reaction, that is, the whole amount of the liquid (A),the liquid (B), and the liquid (C), is mixed. In plain words, it is themolar ratio in the whole amount of the reaction liquid after adding theliquid (B) and the liquid (C) to the liquid (A) (liquid (A)+liquid(B)+liquid (C)).

The molar ratio of water contained in the reaction liquid is, in thecase where the number of moles of the alkoxysilane is 1.0, preferably2.0 to 12.0 mol, and more preferably 3.0 to 6.0 mol. When the molarratio of water is 2.0 mol or more, the amount of unreacted materials canbe reduced. In addition, when the molar ratio of water is 12.0 mol orless, the concentration of silica particles in the obtained silica solcan be enhanced. Incidentally, in the case where the condensate of analkoxysilane used is an N-mer (N represents an integer of 2 or more),the molar ratio of water in the reaction liquid is N times that in thecase of using an alkoxysilane. That is, in the case where the condensateof an alkoxysilane used is a dimer, the molar ratio of water in thereaction liquid is two times that in the case of using an alkoxysilane.

The molar ratio of the alkali catalyst contained in the reaction liquidis, in the case where the number of moles of the alkoxysilane orcondensate thereof is 1.0, preferably 0.1 to 1.0 mol, and morepreferably 0.13 to 0.33.

That is, the amount of alkoxysilane or condensate thereof in a certainamount of alkali catalyst, that is, the molar ratio of the alkoxysilaneor condensate thereof relative to the alkali catalyst (the number ofmoles of the alkoxysilane or condensate thereof (mol)/the number ofmoles of the alkali catalyst (mol)) is preferably 1 to 10, and morepreferably 3 to 8. When the upper limit of the molar ratio is within theabove range, the amount of unreacted materials can be reduced, and alsothe suppressing effect on the formation of fine particles can be furtherenhanced. In addition, when the lower limit of the molar ratio is withinthe above range, the reaction stability can be improved, and also theamount of silica synthesized per batch is further increased, whereby theproductivity can be further enhanced. In the production method accordingto one embodiment of the present invention, even when the amount ofalkoxysilane or condensate thereof used, which serves as a raw material,is increased, the formation of fine particles is suppressed. Therefore,as application conditions for further enhancing the usefulness of thepresent invention, it is particularly preferable that the ratio of thenumber of moles of the alkoxysilane or condensate thereof relative tothe number of moles of the alkali catalyst is adjusted to be within theabove range.

The molar ratio of the total amount of the first and second organicsolvents contained in the reaction liquid is, in the case where thenumber of moles of the alkoxysilane or condensate thereof is as 1.0,preferably 2.0 to 20.0 mol, and more preferably 4.0 to 17.0 mol. Whenthe molar ratio of the organic solvents is 2.0 mol or more, the amountof unreacted materials can be reduced. In addition, when the molar ratioof the organic solvents is 20.0 mol or less, the concentration of silicaparticles in the obtained silica sol can be enhanced.

That is, it is preferable that the molar ratio of the alkoxysilane,water, the alkali catalyst, and the first and second organic solvents inthe reaction liquid is (alkoxysilane):(water):(alkali catalyst):(organicsolvent)=(1.0):(2.0 to 12.0):(0.1 to 1.0):(2.0 to 20.0). In addition, itis preferable that the molar ratio of the alkoxysilane condensate,water, the alkali catalyst, and the first and second organic solvents inthe reaction liquid is, in the case where the alkoxysilane condensate isan N-mer (N represents an integer of 2 or more), (alkoxysilanecondensate):(water):(alkali catalyst):(organic solvent)=(1.0):(2.0×N to12.0×N):(0.1 to 1.0):(2.0 to 20.0).

[Post-Treatment]

In the method for producing a silica sol according to one embodiment ofthe present invention, in addition to the preparation of the abovereaction liquid, the post-treatment described below may also beperformed.

Specifically, the method may further include, after the preparation ofthe reaction liquid and the synthesis reaction, at least one ofdisplacing the organic solvents present in the silica sol with water andconcentrating the silica sol. More specifically, it is possible toperform only the concentration of the silica sol, or perform only thedisplacement of the organic solvents in the silica sol with water. Inaddition, after concentration, the organic solvents in the concentratedliquid may be displaced with water, or, after water displacement, thewater-displaced liquid may be concentrated. Concentration may beperformed several times, and, in such a case, water displacement may beperformed between concentration and concentration. For example, it ispossible that after concentration, the organic solvents in theconcentrated liquid are displaced with water, and the water-displacedliquid is further concentrated.

(Water Displacement)

The method for producing a silica sol according to one embodiment of thepresent invention may further include, after the preparation of thereaction liquid and the synthesis reaction, displacing the organicsolvents contained in the silica sol with water. It is preferable thatwater displacement is performed on a silica sol that has undergoneconcentration (concentrated silica sol).

When the organic solvents in the silica sol are displaced with water, inthe case where ammonia is selected as an alkali catalyst, the pH of thesilica sol can be adjusted to a neutral region. At the same time,unreacted materials contained in the silica sol are removed, whereby awater-displaced silica sol stable over a long period of time can beobtained.

As the method for displacing the organic solvents in the silica sol withwater, a known method can be used. For example, a method in which wateris added while maintaining the liquid amount of the silica sol at orhigher than a certain amount, and displacement is performed by heatingdistillation, can be mentioned. In this case, it is preferable that thedisplacement operation is performed until the liquid temperature and thecolumn top temperature reach the boiling point of water fordisplacement.

As water used in water displacement, in terms of minimizing thecontamination with metal impurities and the like, it is preferable touse pure water or ultrapure water.

In addition, as a method for displacing the organic solvents in thesilica sol with water, a method in which the silica sol is centrifugedto separate silica particles, followed by re-dispersion in water, canalso be mentioned.

(Concentration Step)

The method for producing a silica sol according to one embodiment of thepresent invention may further include, after the preparation of thereaction liquid and the synthesis reaction, further concentrating thesilica sol. Incidentally, the silica sol of this mode also includes amode of a silica sol that has undergone water displacement(water-displaced silica sol).

The method for concentrating a silica sol is not particularly limited,and a known method can be used. For example, a heating concentrationmethod, a membrane concentration method, and the like can be mentioned.

According to the heating concentration method, the silica sol is heatedand concentrated under atmospheric pressure or under reduced pressure,whereby a concentrated silica sol can be obtained.

According to the membrane concentration method, for example, the silicasol can be concentrated by membrane separation using an ultrafiltrationmethod capable of filtrating silica particles. The molecular weightcutoff of the ultrafiltration membrane is not particularly limited, andthe molecular weight cutoff can be selected according to the size ofgenerated particles. The material that forms the ultrafiltrationmembrane is not particularly limited, and polysulfone, polyacrylnitrile,sintering metal, ceramic, carbon, and the like can be mentioned, forexample. The form of the ultrafiltration membrane is not particularlylimited, and a spiral type, a tubular type, a hollow fiber type, and thelike can be mentioned. In the ultrafiltration method, the operatingpressure is not particularly limited, and can be set to be equal to orlower than the working pressure of the ultrafiltration membrane used.

<Silica Sol>

The volume average particle size of silica particles in the silica solproduced by the above production method is not particularly limited, butis preferably 3 to 500 nm, more preferably 5 to 300 nm, still morepreferably 10 to 200 nm, and particularly preferably more than 30 nm and200 nm or less.

Incidentally, as used herein, “fine particles” means particles having aparticle size that is 40% or less of the volume average particle sizedetermined by SEM image analysis and also is 30 nm or less.

The proportion of the number of fine particles is not particularlylimited, and is preferably 10% or less based on the total number ofsilica particles. The proportion is more preferably 5% or less, andstill more preferably 0%, that is, no such particles are observed (lowerlimit: 0%).

As a preferred example of silica particles contained in the silica solproduced by the above production method, particles having a volumeaverage particle size of 10 to 200 nm (more preferably more than 30 nmand 200 nm or less), in which the proportion of the number of fineparticles having a particle size that is 40% or less of the volumeaverage particle size and is 30 nm or less is 10% or less of the totalnumber of the silica particles, for example, can be mentioned.

Here, the volume average particle size of silica particles in the silicasol and the proportion of the number of fine particles based on thetotal number of silica particles can be calculated by the followingmethod, for example. First, from an SEM image taken using a scanningelectron microscope (SEM) SU8000 manufactured by HitachiHigh-Technologies Corporation, 400 silica particles are counted usingimage-analysis particle size distribution software Mac-View Ver. 4(manufactured by Mountech Co., Ltd.), and, based on the Heywood diameter(equivalent-circle diameter), the size of each of particles and theirvolume average particle size are calculated. Subsequently, of 400 silicaparticles, the number of fine particles having a particle size that is40% or less of the volume average particle size determined by SEM imageanalysis and is 30 nm or less is checked. Then, the proportion of thenumber of fine particles relative to the 400 silica particles iscalculated as the proportion (%) of the number of fine particles basedon the total number of silica particles. Incidentally, the details ofthe measurement method will be described in the Examples.

The shape of silica particles in the silica sol is not particularlylimited, and may be spherical or non-spherical.

The concentration of silica particles in the silica sol produced by theabove production method (concentration of silica in the reaction liquid)changes depending on the particle size of the obtained silica particlesand is not particularly limited, but is preferably 5 mass % or more and25 mass % or less, more preferably 7 mass % or more and 20 mass % orless, and still more preferably 9 mass % or more and 15 mass % or less.

The pH of the silica sol produced by the production method according toone embodiment of the present invention is not particularly limited, butis preferably 7.0 to 13.0, and more preferably 8.0 to 12.0.

According to the production method according to one embodiment of thepresent invention, the total content of metal impurities contained inthe silica sol, for example, metal impurities such as Al, Ca, B, Ba, Co,Cr, Cu, Fe, Mg, Mn, Na, Ni, Pb, Sr, Ti, Zn, Zr, U, and Th, is notparticularly limited, but is preferably 1 ppm or less. Embodiments ofthe present invention have been described in detail. However, they areillustrative and exemplary, but not restrictive, and the scope of thepresent invention is clearly to be determined from the attached Claims.

The present invention encompasses the following embodiments and modes:

-   1. A method for producing a silica sol, including synthesizing a    silica sol by, in a reaction liquid containing an alkoxysilane or a    condensate thereof, water, and an alkali catalyst, allowing the    alkoxysilane or condensate thereof to react with the water in the    presence of the alkali catalyst, wherein

the alkali catalyst is not additionally supplied after the start of thesynthesis until the finish time of the synthesis, and

during 90% or more of the time between when 5 minutes have elapsed fromthe time point when the electrical conductivity of the reaction liquidreaches a local maximum for the first time from the start of thereaction and the finish time of the synthesis, the proportion of thevalue of the electrical conductivity of the reaction liquid is more than90% relative to the value of the electrical conductivity at the timewhen 5 minutes have elapsed from the time point when the local maximumis reached;

-   2. The method for producing a silica sol according to 1. above,    including, during at least part of the time between the start time    of the synthesis and the finish time of the synthesis, lowering the    temperature of the reaction liquid stepwise or continuously;-   3. The method for producing a silica sol according to 2. above,    including, during the entire time between when 5 minutes have    elapsed from the time point when the electrical conductivity of the    reaction liquid reaches a local maximum for the first time and the    finish time of the synthesis, lowering the temperature of the    reaction liquid continuously;-   4. The method for producing a silica sol according to any one of 1.    to 3. above, wherein, relative to the total amount of the    alkoxysilane or condensate thereof added between when 5 minutes have    elapsed from the time point when the electrical conductivity of the    reaction liquid reaches a local maximum for the first time and the    finish time of the synthesis, the proportion (%) of the amount of    the alkoxysilane or condensate thereof added during this period in    the state where the proportion of the value of the electrical    conductivity of the reaction liquid is more than 90% relative to the    value of the electrical conductivity at the time when 5 minutes have    elapsed from the time point when the electrical conductivity of the    reaction liquid reaches a local maximum for the first time is 90    mass % or more;-   5. The method for producing a silica sol according to any one of 1.    to 4. above, wherein the alkoxysilane or condensate thereof is added    at a constant addition rate between the start time of the synthesis    and the finish time of the synthesis;-   6. The method for producing a silica sol according to any one of 1.    to 5. above, wherein, relative to the total amount of the    alkoxysilane or condensate thereof added between the start time of    the synthesis and the finish time of the synthesis, the proportion    (%) of the amount of the alkoxysilane or condensate thereof added    between the start time of the synthesis and the time point when the    electrical conductivity of the reaction liquid reaches a local    maximum for the first time is less than 20 mass %;-   7. The method for producing a silica sol according to any one of 1.    to 6. above, including preparing the reaction liquid by mixing:

a liquid (B) containing the alkoxysilane or condensate thereof and asecond organic solvent; and

a liquid (C1) containing the water and having a pH of 5.0 or more andless than 8.0; with

a liquid (A) containing the alkali catalyst, the water, and a firstorganic solvent;

-   8. The method for producing a silica sol according to any one of 1.    to 6. above, including preparing the reaction liquid by mixing:

a liquid (B) containing the alkoxysilane or condensate thereof and asecond organic solvent; and

a liquid (C2) containing the water and not containing the alkalicatalyst; with

a liquid (A) containing the alkali catalyst, the water, and a firstorganic solvent;

-   9. The method for producing a silica sol according to any one of 1.    to 8. above, wherein

silica particles included in the silica sol have a volume averageparticle size of 10 nm or more and 200 nm or less, and

the proportion of the number of fine particles having a particle sizethat is 40% or less of the volume average particle size and is 30 nm orless is 10% or less of the total number of the silica particles.

-   10. The method for producing a silica sol according to any one of 1.    to 9. above, wherein the alkali catalyst is ammonia.

EXAMPLES

The present invention will be described in further detail using thefollowing examples and comparative examples. However, the technicalscope of the present invention is not limited only to the followingexamples. Incidentally, unless otherwise noted, “%” and “parts” mean “%by mass” and “parts by mass”, respectively. In addition, in thefollowing Examples, unless otherwise noted, the operations wereperformed under conditions of room temperature (25° C.)/relativehumidity 40 to 50% RH.

<Preparation of Silica Sol>

Example 1

(Preparation of Silica Sol)

To a liquid (A) obtained by mixing 97 g of water and 58 g of 29 mass %aqueous ammonia with 976 g of methanol, the addition of a liquid (B)obtained by dissolving 190 g of methanol and 506 g of tetramethoxysilane(TMOS) and a liquid (C) which is 119 g of pure water (pH=7.85) wasstarted to prepare a reaction liquid. The synthesis reaction was allowedto proceed, then the addition was finished to complete the synthesisreaction, thereby giving a silica sol.

Here, in the preparation of the reaction liquid, the temperature of eachliquid before mixing was maintained at 35° C., and the whole amount ofthe liquid (B) and the liquid (C) was simultaneously added to the liquid(A) each at a constant addition rate over 75 minutes while adjusting thetemperature such that the temperature of the reaction liquid decreasedfrom 35° C., which is the initial reaction temperature at the start timeof the addition of the liquid (B) and the liquid (C) to the liquid (A)(start time of the synthesis), to 24.5° C., which is the final reactiontemperature at the finish time of the addition (finish time of thesynthesis). Here, it was confirmed that of the addition time of 75minutes, from the start time of the addition to when 6.5 minutes hadelapsed, the temperature increased due to the reaction heat.Subsequently, it was confirmed that during 68.5 minutes from when 6.5minutes had elapsed from the start time of the addition to the finish ofthe synthesis, the temperature continuously decreased.

In addition, the pH of the liquid (C) was measured with a desktop pHmeter (Model No.: F-72) manufactured by HORIBA, Ltd.

(Concentration of Silica Sol)

Under atmospheric pressure and at a temperature where the silica solturned into a boiling state, the silica sol obtained above was heateduntil the concentration of silica reached 20 mass %, thereby giving aheat-concentrated silica sol.

(Water Displacement of Silica Sol)

While maintaining the liquid amount of the heat-concentrated silica solobtained above at or higher than a certain level, pure water was added,followed by heating distillation, whereby the organic solvent in theheat-concentrated silica sol was displaced with pure water, therebygiving a water-displaced silica sol. Here, the heating distillation wasperformed as follows. Under atmospheric pressure and in a boiling state,heating was performed until the liquid temperature stopped increasing,and 1,780 g of pure water was added to the whole amount of the reactionliquid, followed by distillation.

Comparative Example 1

The preparation of a silica sol, the heating concentration of the silicasol, and the water displacement of the silica sol were performed in thesame manner as in Example 1, except that in the preparation of a silicasol in Example 1, the temperature of the reaction liquid at the starttime of the addition (start time of the synthesis) was set at 35° C., adecrease in the temperature between the start time of the addition andthe finish time of the addition (finish time of the synthesis) wassuppressed, and, while adjusting the temperature to be as constant aspossible, the liquid (B) and the liquid (C) were added to the liquid(A), thereby giving a silica sol.

(Measurement of Temperature of Reaction Liquid and ElectricalConductivity of Reaction Liquid)

In the preparation of a silica sol described above, using Lacom TesterpH & Conductivity Meter PCWP300 (manufactured by Universal Technics Co.,Ltd.), the electrodes of this device were immersed in the reactionliquid, and the temperature of the reaction liquid and the electricalconductivity (μS/cm) of the reaction liquid were measured every 0.25minutes from the start time of the addition (start time of thesynthesis). The rate of change in electrical conductivity was calculatedbased on the following formula.(Rate of change in electrical conductivity)(%)=[(electrical conductivityat a noted time between when 5 minutes have elapsed from the first localmaximum and the finish time of the synthesis)/(electrical conductivityat the time when 5 minutes have elapsed from the first localmaximum)]×100  [Equation 1]

Incidentally, in Example 1 and Comparative Example 1, the firstlocal-maximum time point was when 1.5 minutes had elapsed from the startof the synthesis in each case, and the time point when 5 minutes hadelapsed from the first local maximum was when 6.5 minutes have elapsedfrom the start of the synthesis in each case.

With respect to the silica sol production methods according to Example 1and Comparative Example 1, the amount of raw materials used and thereaction conditions are summarized in Table 1 below. Incidentally, inTable 1 below, the minimum valve of the rate of change in electricalconductivity between when 5 minutes have elapsed from the first localmaximum and the finish time of the synthesis is shown as “electricalconductivity maintenance range (%)”.

Reaction liquid temperature-reaction time graphs in the productionmethods according to Example 1 and Comparative Example 1 are shown inFIG. 1A and FIG. 1B, and electrical conductivity-reaction time graphs inthe production methods according to Example 1 and Comparative Example 1are shown in FIG. 2A and FIG. 2B, respectively. Here, FIG. 1A shows thereaction liquid temperature-reaction time in Example 1, and FIG. 1Bshows the reaction liquid temperature-reaction time in ComparativeExample 1. In addition, FIG. 2A shows the electricalconductivity-reaction time in Example 1, and FIG. 2B shows theelectrical conductivity-reaction time in Comparative Example 1.

In addition, in Example 1 and Comparative Example 1, the liquid (B) andthe liquid (C) is added at a constant addition rate to the liquid (A).

Here, relative to the time from when 5 minutes have elapsed from thefirst local maximum until the finish time of the synthesis of a silicasol, the proportion (%) of the time during which the state where theproportion of the value of the electrical conductivity of the reactionliquid is more than 90% relative to the value of the electricalconductivity at the time when 5 minutes have elapsed from the firstlocal maximum is presented is shown in Table 1 below as “proportion oftime with no decrease in electrical conductivity”.

In addition, the proportion (%) of, relative to the time from when 5minutes have elapsed from the first local maximum until the finish timeof the synthesis of a silica sol, the time during which the state wherethe proportion of the value of the electrical conductivity of thereaction liquid is more than 90% relative to the value of the electricalconductivity at the time when 5 minutes have elapsed from the firstlocal maximum is presented is equal to the following proportion. Thatis, it is equal to the proportion (%) of, relative to the total amountof the alkoxysilane or condensate thereof added between when 5 minuteshave elapsed from the first local maximum and the finish time of thesynthesis of a silica sol, the amount of the alkoxysilane or condensatethereof added during this period in the state where the proportion ofthe value of the electrical conductivity of the reaction liquid is morethan 90% relative to the value of the electrical conductivity at thetime when 5 minutes have elapsed from the first local maximum. Theproportion of the amount of alkoxysilane or condensate thereof is shownin Table 1 below as “addition proportion of TMOS in the state with nodecrease in electrical conductivity”.

In addition, the ratio of the number of moles of TMOS in the liquid (B)to the number of moles of ammonia in the liquid (A) in the productionmethods according to Example 1 and Comparative Example 1 is shown as“molar ratio of TMOS to ammonia” in Table 2. In the calculation, themolar mass of ammonia was defined as 17 g/mol, and the molar mass ofTMOS was defined as 152.25 g/mol.

Further, the concentration of silica (mass %) in the reaction liquidcalculated by the following equation in the case where all TMOS is usedfor the synthesis of silica is shown in Table 2. In the calculation, themolar mass of ammonia was defined as 17 g/mol, the molar mass of TMOSwas defined as 152.25 g/mol, and the molar mass of silica was defined as60.1 g/mol.(Concentration of silica in the reaction liquid)(%)={[(the number ofmoles of TMOS in the liquid(B))(mol)×(molar mass ofsilica(SiO₂))(g/mol)]/(total mass(g) of the liquid(A), the liquid(B),and the liquid(C))}×100  [Equation 2]<Evaluation of Silica Sol>(Calculation of Volume Average Particle Size of Silica Particles)

The silica sol obtained above was dispersed in alcohol, then dried, andinstalled under a scanning electron microscope (SEM) SU8000(manufactured by Hitachi High-Technologies Corporation), followed byelectron beam irradiation at 5.0 kV. At a magnification of 50,000, SEMimages of several visual fields were taken such that the total number ofsilica particles was 400 or more.

Next, from the SEM images taken above, 400 silica particles were countedusing image-analysis particle size distribution software Mac-View Ver. 4(manufactured by Mountech Co., Ltd.). Subsequently, based on the Heywooddiameter (equivalent-circle diameter), the particle size of each ofsilica particles and their volume average particle size were calculated.

(Calculation of Proportion of the Number of Fine Particles in SilicaParticles)

In the above calculation of the volume average particle size of silicaparticles, of the 400 silica particles, the number of fine particleshaving a particle size that is 40% or less of the volume averageparticle size determined by SEM image analysis and is 30 nm or less waschecked. Then, the proportion of the number of fine particles relativeto the 400 silica particles was calculated as the proportion (%) of thenumber of fine particles based on the total number of silica particles.

FIG. 3A and FIG. 3B show SEM images of silica particles contained insilica sols produced by the production methods according to Example 1and Comparative Example 1. Here, FIG. 3A shows an SEM image of silicaparticles in Example 1, and FIG. 3B shows an SEM image of silicaparticles in Comparative Example 1. Incidentally, in FIGS. 3A and 3B, 1indicates silica fine particles, and 2 indicates silica particles thatare not fine particles (main particles).

The evaluation results of the silica sols produced by the silica solproduction methods according to Example 1 and Comparative Example 1 areshown in Table 3 below.

TABLE 1 Amount of Raw Materials Used and Reaction Conditions in SilicaSol Production Addition proportion of Proportion of TMOS in the Liquid(A) Liquid Initial Final Electrical time with no state with no [g]Liquid (B) (C) reaction reaction Reac- conductivity decrease in decreasein 29 mass % [g] [g] temper- temper- tion maintenance electricalelectrical Meth- Pure aqueous Meth- Pure ature ature time rangeconductivity conductivity anol water ammonia TMOS anol water [° C.] [°C.] [min] [%] [%] [%] Example 1 976 97 58 506 190 119 35.0 24.5 75 96%100 100 Comparative 976 97 58 506 190 119 35.0 35.9 75 73% 20 20 Example1

TABLE 2 (Table 2) TMOS Amount Relative to Ammonia, and SilicaConcentration in Reaction Liquid Ratio of the number of moles of Silicaconcentration in TMOS to the number of moles of the reaction liquidammonia [mass %] Example 1 3.4 10.2 Comparative 3.4 10.2 Example 1

TABLE 3 (Table 3) Silica Sol Evaluation Results Particle size of 40%Proportion of the Volume average volume average number of fine particlesize [nm] particle size [nm] particles [%] Example 1 74 29 0 ComparativeExample 1 74 29 29

From the results shown above in Table 1 and Table 3, it was confirmedthat in the production method according to Example 1 where theelectrical conductivity maintenance range is 96%, visually observable,obvious fine particles are not formed, and a silica sol with highhomogeneity can be produced. Meanwhile, it was confirmed that in theproduction method according to Comparative Example 1 where theelectrical conductivity maintenance range is 73%, visually observable,obvious fine particles are formed, the amount of fine particles formedis also large, and the produced silica sol had poor homogeneity.

In addition, from the results in Table 1 and Table 3, between theproduction method according to Example 1 and the production methodaccording to Comparative Example 1, although the concentration of silicain the reaction liquid is the same, fine particles were not formed inthe production method according to Example 1. From this, it wasconfirmed that in the production method according to Example 1, fineparticles are not formed even when the amount of alkoxysilane used isincreased, and a homogeneous silica sol can be produced, making itpossible to improve the amount of silica produced per batch, leading tohigher productivity.

This application is based on Japanese Patent Application Number No.2019-064654 filed on Mar. 28, 2019, the contents of which are entirelyincorporated herein by reference.

REFERENCE SIGNS LIST

1: Silica fine particles,

2: Silica particles that are not fine particles (main particles).

What is claimed is:
 1. A method for producing a silica sol, comprisingsynthesizing a silica sol by, in a reaction liquid containing analkoxysilane or a condensate thereof, water, and an alkali catalyst,allowing the alkoxysilane or condensate thereof to react with the waterin the presence of the alkali catalyst, wherein the alkali catalyst isnot additionally supplied after a start of the synthesis until a finishtime of the synthesis, during 90% or more of the time between when 5minutes have elapsed from a time point when an electrical conductivityof the reaction liquid reaches a local maximum for a first time fromstart of a reaction and the finish time of the synthesis, a proportionof a value of the electrical conductivity of the reaction liquid is morethan 90% relative to the value of the electrical conductivity at thetime when 5 minutes have elapsed from the time point when the localmaximum is reached, and the method further comprises, during at leastpart of the time between a start time of the synthesis and the finishtime of the synthesis, lowering a temperature of the reaction liquidstepwise or continuously.
 2. The method for producing a silica solaccording to claim 1, comprising, during an entire time between when 5minutes have elapsed from the time point when the electricalconductivity of the reaction liquid reaches a local maximum for thefirst time and the finish time of the synthesis, lowering thetemperature of the reaction liquid continuously.
 3. The method forproducing a silica sol according to claim 1, wherein, relative to atotal amount of the alkoxysilane or condensate thereof added betweenwhen 5 minutes have elapsed from the time point when the electricalconductivity of the reaction liquid reaches a local maximum for thefirst time and the finish time of the synthesis, the proportion (%) ofthe amount of the alkoxysilane or condensate thereof added during thisperiod in a state where the proportion of the value of the electricalconductivity of the reaction liquid is more than 90% relative to thevalue of the electrical conductivity at the time when 5 minutes haveelapsed from the time point when the electrical conductivity of thereaction liquid reaches a local maximum for the first time is 90 mass %or more.
 4. The method for producing a silica sol according to claim 1,wherein the alkoxysilane or condensate thereof is added at a constantaddition rate between the start time of the synthesis and the finishtime of the synthesis.
 5. The method for producing a silica solaccording to claim 1, wherein, relative to a total amount of thealkoxysilane or condensate thereof added between the start time of thesynthesis and the finish time of the synthesis, the proportion (%) of anamount of the alkoxysilane or condensate thereof added between the starttime of the synthesis and the time point when the electricalconductivity of the reaction liquid reaches a local maximum for thefirst time is less than 20 mass %.
 6. The method for producing a silicasol according to claim 1, comprising preparing the reaction liquid bymixing: a liquid (B) containing the alkoxysilane or condensate thereofand a second organic solvent; and a liquid (C1) containing the water andhaving a pH of 5.0 or more and less than 8.0; with a liquid (A)containing the alkali catalyst, the water, and a first organic solvent.7. The method for producing a silica sol according to claim 1,comprising preparing the reaction liquid by mixing: a liquid (B)containing the alkoxysilane or condensate thereof and a second organicsolvent; and a liquid (C2) containing the water and not containing thealkali catalyst; with a liquid (A) containing the alkali catalyst, thewater, and a first organic solvent.
 8. The method for producing a silicasol according to claim 1, wherein silica particles included in thesilica sol have a volume average particle size of 10 nm or more and 200nm or less, and the proportion of a number of fine particles having aparticle size that is 40% or less of the volume average particle sizeand is 30 nm or less is 10% or less of a total number of the silicaparticles.
 9. The method for producing a silica sol according to claim1, wherein the alkali catalyst is ammonia.